Focusing microprobes, or “laser scalpels,” are used in many applications, such as medical applications, where an intense laser beam needs to be delivered to and tightly focused at the surface of specimen or tissue. There are several important requirements associated with such focusing microprobes.
First, in many applications, the laser radiation needs to be delivered using a flexible waveguide system or the like, where the laser beam is isolated from the surrounding medium. For example, this is required in some surgical procedures performed in sensitive tissues, such as brain surgeries, ultraprecise intraocular surgeries, and the like. In particular, this is important when using wavelengths that are readily absorbed by the sensitive tissues, such as the emission wavelength of an Er:YAG laser (λ=2.94 μm). Flexible optical delivery is also required in applications where the path of the optical beams needs to be curved in space because of geometrical or mechanical restrictions, such as in biomedical spectroscopy applications and the like. The use of flexible optical delivery systems typically leads to additional optical losses and to reduced quality of the optical beams.
Second, for focusing microprobes, systemic optical losses must be minimized. There are many factors contributing to optical losses in such systems, including coupling losses between a source and the flexible optical delivery system, coupling losses in a focusing tip, absorption, scattering and reflection, and other factors. In addition, a large power transmission is typically required in applications such as ultraprecise laser surgery, micro-welding, bonding, and surface patterning, compounding the systemic optical loss problem.
Third, for focusing microprobes, the diameter of the focused laser beam at the tip of the microprobe must be minimized in order to provide more localized laser action. In the case of ultraprecise laser surgery, for example, this is required to minimize the size of ablation craters in the tissue.
Fourth, in many applications, the microprobe must be able to operate in contact or in near contact with the tissue or other specimen. In some applications, this is required due to strong absorption of the laser radiation by the surrounding medium. It can also make the surgery or other procedure easier and more efficient to perform.
Finally, the design of such microprobes involves a trade-off between the total transmitted power and the diameter of the focal spot. One approach to the design of such microprobes is based on using multi-modal delivery systems. These multi-modal delivery systems usually guide hundreds of modes and tend to favor high total transmitted power at the expense of focal spot size. Another approach to the design of such microprobes is based, in part, on using single-mode or few-mode delivery components. The definition of a few-mode system is not precise, but for the purposes of the present invention, fibers with 3<V<10, where V=2πα(NA)/λ, are considered few-mode. In this definition, a is the radius of the core, NA is the numerical aperture, and λ is the wavelength of light. The condition for single-mode operation is V<2.405. The definition of few-mode fibers implies that the core of the waveguiding system is slightly wider than that required by the single-mode cut-off condition. As a result, a few modes are guided by the system, however, this number is very limited, typically in 2-6 range. The main focus of the present invention is on single-mode systems. However, the general approach can be extended to few-mode systems. Single and few-mode systems usually allow much better focusing as compared to multi-mode systems, but they require single-mode laser radiation sources for efficient operation. The classical single-mode delivery component is represented by a conventional single-mode fiber. However, such fibers are not well developed for the mid-IR ranges of wavelengths required for some applications, such as a contact laser surgery.
Thus, the present invention utilizes a flexible laser radiation delivery system, i.e. a hollow-core fiber. This delivery system has sufficiently large core diameter, simplifying the coupling of single-mode or few-mode laser emissions into such cores. On the other hand, as a delivery system, it still operates in a close to single-mode regime that is important for achieving tight beam focusing. The system of the present invention provides a unique solution for flexible laser radiation delivery combined with tight focusing in cases where conventional single-mode fibers are not available or may not be used. The system of the present invention allows for a continuous single-mode (or few-mode) design all the way from the laser radiation source to actual application in tissue or other media. This is novel and this design provides critical advantages over other designs in cases where compact and portable single-mode laser radiation sources must be used with very high efficiency for producing tightly focused beams.